Selection on reproductive characters: floral morphology in Asclepias syriaca

Abstract

Pollinia insertions and removals represent a convenient index of male and female fertilities in the Asclepiadaceae. This fact, in conjunction with regression analysis, allows estimation and comparison of gender-specific selection acting on floral characters in Asclepias syriaca. Univariate regression analysis indicates statistically significant selection on minute aspects of floral morphology through the male and especially female reproductive pathways. Analyses using multiple regression to correct for overall plant size do not qualitatively change the univariate results. The relatively large number of statistically significant selection gradients suggests active adaptation of floral morphological features to the pollinator fauna. Selection through female function differs significantly from selection on the same trait through male function for four traits, emphasizing an antagonism between genders in hermaphrodites. This antagonism and the absence of selection of the antagonistic traits through net fertility are consistent with an equilibrium quantitative genetic perspective, while greater opportunity for selection through female function is counter to expectations based on Bateman's principle of sexual selection.